drm/i915/dp: Introduce is_cpu_edp()

The obvious counterpart to is_pch_edp().  Convert existing instances of
the idiom to the new routine.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>